1. Field of the Invention
This invention relates to automatic exposure control devices suited to be used in video cameras, etc.
2. Description of the Related Art
In video cameras, to keep exposures always at an optimum, provision is made for automatically controlling the exposure.
The conventional automatic exposure control devices generally have functions of controlling the iris of the optical system so that the video signal at the output takes a constant level and of controlling the level of the video signal. As the light measuring circuit for controlling the iris and the video signal level, it has been the common practice to use a circuit for averaging the luminance of the entirety of an image sensing plane. In lighting situations having a large luminance difference between a subject and its background, however, for the backlighting case, the subject is blackened. For the frontlighting case, the subject is washed out. In both cases, unnatural pictures result. To avoid this, a method has been proposed that a portion of the image sensing plane, for example, a central portion in which the subject falls with a high probability, is framed for light measurement, and is weightily measured for the luminance of the interior of that frame. In this light measuring method, the position of the light measuring frame is fixed relative to the image sensing plane. It will hereinafter be called the "fixed-frame weighted light measurement" method.
FIG. 1 in block diagram shows the construction and arrangement of the prior known automatic exposure control device employing the fixed-frame weighted light measurement method in the video camera commonly available for home use. Light entering through a photographic optical system 10 is regulated in intensity by an iris 12, before reaching an image sensor 14. The device further comprises a buffer amplifier 16, an AGC (Automatic Gain Control) circuit 18, a light measuring circuit 20 for controlling the iris 12, an iris drive circuit 22 responsive to the output of the light measuring circuit 20 for moving the iris 12, another light measuring circuit 24 for controlling the gain of the AGC circuit 18, and a frame signal generating circuit 26 for producing a frame signal indicating the light measuring frame set by the first and second light measuring circuits 20 and 24. The video signal output appears at a terminal 28. A composite synchronizing signal is supplied from a terminal 30.
As is understandable from the above, the first light measuring circuit 20 controls the iris 12 by the iris drive circuit 22 so as to make constant the output level of the image sensor 14, while the second light measuring circuit 24 controls the gain of the AGC circuit 18 so as to make constant the output signal level of the AGC circuit 18.
FIG. 2 shows an example of construction of the first light measuring circuit 20. The second light measuring circuit 24 also is similar in construction to it. The circuit 20 comprises an analog switch 31, two low-pass filters (LPF) 32 and 34, two resistors 36 and 38 and a buffer 40. The output of the buffer 16, or the video signal, is applied directly to the LPF 32 and, through the analog switch 31, to the LPF 34. The analog switch 31 is opened or closed under the control of the frame signal output from the frame signal generating circuit 26. For that part of the video signal which corresponds to the interior of the light measuring frame, the analog switch 31 comes into a closed state. In other words, the LPF 32 outputs an average signal on the whole image sensing plane, while the LPF 34 outputs an average signal in part of the image sensing plane, or in the light measuring frame. These average signals after having been weighted by the resistors 36 and 38 respectively are then added together and output from the buffer 40. As the weight laid on the output of the first LPF 32 increases, the device approaches the whole-image-sensing-plane average light measurement mode. As the weight laid on the output of the second LPF 34 increases, the device approaches the light-measuring-frame weighted light measurement mode.
However, the above-described fixed-frame weighted light measurement method has the following problems. That is, the light measuring frame fails to be of the appropriate size to get only information about the whole subject. Since, in that method, however, the light measuring frame has its position fixed relative to the image sensing plane and its size remains constant, good versatility of the exposure control to all photographic situations cannot be realized. For example, in outdoor photography, the backlighting situation with the sky in the upper marginal zone of the image sensing plane is very often encountered. With regard to this, to avoid blackening of a subject as its luminance is weightily measured, the light measuring frame is desired to be wide in the lateral direction and large in size as shown in FIG. 3(a). For another subject of relatively small size as shown in FIG. 3(b), however, when backlighting occurs, the light measuring frame is partly occupied by the background of high luminance so that the average luminance rises greatly. Hence, blackening of the subject results.
Also, when over-frontlighting occurs as shown in FIG. 3(c), the background of low luminance enters the light measuring frame so that the average luminance is lowered, causing the subject to be washed out.